This invention relates generally to protecting metals from a corrosive environment, and more specifically to protecting copper-aluminum heat exchangers for use in air conditioners.
Galvanic corrosion occurs when two dissimilar metals make contact with one another in the presence of an electrolyte thereby forming a galvanic couple. The more noble metal (higher on the galvanic series) provides the surface area for the reduction reaction and the less noble metal (lower on the galvanic series) corrodes in an oxidation process. The oxidation occurs in the greatest amount at the interface of the two metals but may also occur at some distance away from the actual interface. In coastal regions, the most common electrolyte is salt water in the air. A fine salt water mist may be blown inland for up to fifty miles from the coast. Sulfur dioxide from industrial pollution also creates an electrolyte when it combines with moisture in the air.
A common method of preventing galvanic corrosion has been to coat the exposed surfaces of the metals with various types of paint. These protective coatings have met with only limited success for a number of reasons. The main problem with coatings is that their effectiveness at preventing corrosion is degraded by exposure to the environment such as ultraviolet light and acid rain. Another common problem is that the coating materials often do not adhere well to the metal substrates and eventually flake off or erode away exposing the metal substrates. Moreover, such protective coatings are somewhat porous and allow the electrolyte to penetrate the surface of the substrates and connect the galvanic couple. In addition, the application of protective coatings to the surfaces of certain articles can negatively affect their performance.
Attempts have been made, with varying degrees of success, to coat conventional copper-aluminum heat exchangers with various materials in an effort to extend the useful life of the unit. These coating materials oftentimes reduce the heat transfer capability of the unit, exhibit poor adhesion properties and fail to penetrate into all the areas that might be exposed to a hostile environment.
In general the present invention provides an advanced galvanic corrosion protection method. In accordance with the present invention, in an article made of two metals, one being more noble than the other, the outer surface of the more noble metal is treated with a metal which is galvanically compatible with the less noble metal to form a protective layer between the two dissimilar metals which prevents the reduction reaction of the galvanic couple from occurring. The invention greatly reduces the oxidation reduction process which occurs when two dissimilar metals are in contact with one another in the presence of an electrolyte.